The Sun-grazing asteroid, Phaethon, has betrayed its true nature by showing a comet-like tail of dust particles blown backwards by radiation pressure from the Sun. Unlike a comet, however, Phaethon’s tail doesn’t arise through the vaporization of an icy nucleus. During its closest approach to the Sun, researchers believe that Phaethon becomes so hot that rocks on the surface crack and crumble to dust under the extreme heat. The findings will be presented by David Jewitt on Tuesday 10 September at the European Planetary Science Congress (EPSC) 2013 in London.

Most meteor showers arise when the Earth ploughs through streams of debris released from comets in the inner solar system. The Geminids, which grace the night sky annually in December, are one of the best known and most spectacular of the dozens of meteor showers. However, astronomers have known for 30 years that the Geminids are not caused by a comet but by a 5 km diameter asteroid called (3200) Phaethon.

Until recently, though, and much to their puzzlement, astronomer’s attempts to catch Phaethon in the act of throwing out particles all ended in failure. The tide began to turn in 2010 when Jewitt and colleague, Jing Li, found Phaethon to be anomalously bright when closest to the Sun. The key to success was their use of NASA’s STEREO Sun-observing spacecraft. Phaethon at perihelion appears only 8 degrees (16 solar diameters) from the sun, making observations with normal telescopes impossible. Now, in further STEREO observations from 2009 and 2012, Jewitt, Li and Jessica Agarwal have spotted a comet-like tail extending from Phaethon.

“The tail gives incontrovertible evidence that Phaethon ejects dust,” said Jewitt. ‘That still leaves the question: why? Comets do it because they contain ice that vaporizes in the heat of the Sun, creating a wind that blows embedded dust particles from the nucleus. Phaethon’s closest approach to the Sun is just 14 per cent of the average Earth-Sun distance (1AU). That means that Phaethon will reach temperatures over 700 degrees Celsius – far too hot for ice to survive.”

The team believes that thermal fracture and desiccation fracture (formed like mud cracks in a dry lake bed) may be launching small dust particles that are then picked up by sunlight and pushed into the tail. While this is the first time that thermal disintegration has been found to play an important role in the Solar System, astronomers have already detected unexpected amounts of hot dust around some nearby stars that might have been similarly-produced.

So, is Phaethon an asteroid or a comet? Asteroids and comets derive from entirely different regions of the solar system; asteroids from between Mars and Jupiter (roughly 2 to 3.5 AU) and comets from the frigid trans-Neptunian realms (30 AU and beyond).

“By the shape of its orbit, Phaethon is definitely an asteroid. But by ejecting dust it behaves like a ‘rock comet’,” said Jewitt.

EPSC is the major European meeting on planetary science. EPSC 2013 is taking place at University College London (UCL) from Sunday 8 September to Friday 13 September 2013. It is the first time that the Congress has been held in the UK. The 2013 programme includes around 75 sessions and workshops. Details of the Congress and a full schedule of EPSC 2013 scientific sessions and events can be found at the official website: http://www.epsc2013.eu/

EPSC 2013 is organised by Europlanet, UCL and Copernicus Meetings and the event is sponsored by the UK Space Agency, UCL, Astrium and the Science and Technology Facilities Council.

To celebrate EPSC coming to London, a ‘Festival of the Planets’ has been organised across the Capital in collaboration with partners including the Baker Street Irregular Astronomers, the Bloomsbury Theatre, the British Astronomical Association, the British Interplanetary Society, the Natural History Museum, the Open University, Queen Mary University of London, the Royal Astronomical Society, Royal Museums Greenwich and University College London. More information about the events can be found at:

Europlanet is a network of planetary scientists, whose aim is to bring together the disparate European community so that Europe can play a leading role in space exploration. Europlanet’s activities complement the mission activities of the European Space Agency through field work at planetary-analogue terrains on Earth, laboratory measurements, computer modelling and observations from ground-based telescopes. Founded in 2002 and funded by the European Commission from 2005-2012, Europlanet has evolved into a community-based organisation that will carry on this work and plan for future missions and mission support.

www.europlanet-eu.org

About UCL (University College London)

Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender and the first to provide systematic teaching of law, architecture and medicine.

UCL is among the world's top universities, as reflected by its performance in a range of international rankings and tables. According to the Thomson Scientific Citation Index, UCL is the second most highly cited European university and the 15th most highly cited in the world.

UCL has nearly 27,000 students from 150 countries and more than 9,000 employees, of whom one third are from outside the UK. The university is based in Bloomsbury in the heart of London, but also has two international campuses – UCL Australia and UCL Qatar. Its annual income is more than £800 million.